Add flow control between renderer and GPU processes, and, on Mac OS X,...

chrome/renderer/ggl/ggl.cc

Index: chrome/renderer/ggl/ggl.cc
===================================================================
--- chrome/renderer/ggl/ggl.cc	(revision 67289)
+++ chrome/renderer/ggl/ggl.cc	(working copy)
@@ -34,6 +34,12 @@
 // creation attributes.
 const int32 kTransferBufferSize = 1024 * 1024;
 
+// TODO(kbr) / TODO(apatrick): determine the best number of pending frames
+// in the general case. On Mac OS X it seems we really want this to be 1,
+// because otherwise the renderer process produces frames that do not
+// actually reach the screen.

[Nico, 2010/11/24 23:43:29]

True, but the browser process would show the latest of the N frames the renderer
produces, not the first. So it's not all wasted work, it reduces display
latency.

[Ken Russell (switch to Gerrit), 2010/11/25 01:02:08]

Correct.
That's possible. More investigation is needed. In Safari, where Core Animation's
built-in rate limiting mechanisms are in use, Chris Rogers' fill-limited test
case reaches a steady state of 60 FPS. If we increase this number here to 2 then
Chromium claims 120 FPS, but the browser is only displaying 60 FPS. In that
situation we are definitely doing more rendering work than desired. We may need
to do some sort of adaptive throttling to get the best behavior.

+const int kMaxFramesPending = 1;
+
 // Singleton used to initialize and terminate the gles2 library.
 class GLES2Initializer {
  public:
@@ -132,6 +138,8 @@
   gpu::gles2::GLES2Implementation* gles2_implementation_;
   gfx::Size size_;
 
+  int32 swap_buffer_tokens_[kMaxFramesPending];
+
   Error last_error_;
 
   DISALLOW_COPY_AND_ASSIGN(Context);
@@ -147,6 +155,8 @@
       gles2_implementation_(NULL),
       last_error_(SUCCESS) {
   DCHECK(channel);
+  for (int i = 0; i < kMaxFramesPending; ++i)
+    swap_buffer_tokens_[i] = -1;
 }
 
 Context::~Context() {
@@ -376,7 +386,18 @@
   if (command_buffer_->GetLastState().error != gpu::error::kNoError)
     return false;
 
+  // Throttle until there are not too many frames pending.
+  if (swap_buffer_tokens_[0] != -1) {
+    gles2_helper_->WaitForToken(swap_buffer_tokens_[0]);
+  }
+
   gles2_implementation_->SwapBuffers();
+
+  // Insert a new token to throttle against for this frame.
+  for (int i = 0; i < kMaxFramesPending - 1; ++i)
+    swap_buffer_tokens_[i] = swap_buffer_tokens_[i + 1];
+  swap_buffer_tokens_[kMaxFramesPending - 1] = gles2_helper_->InsertToken();
+
   return true;
 }